Tape servo track write compensation

ABSTRACT

Setting a servo write head spacing for writing servo tracks on a magnetic tape in a manufacturing environment. One or more processors receive one or more environmental condition measurements of the manufacturing environment. The processors determine a spacing for a pair of servo write heads based at least on a nominal spacing for the pair of servo tracks, and a product of a difference between the nominal environmental conditions associated with the nominal spacing and the corresponding environmental condition measurements of the manufacturing environment, and coefficients of expansion for the magnetic tape. Spacing for the pair of servo write heads is set to the determined spacing, such that the spacing of the pair of servo write heads is substantially the nominal spacing when the one or more manufacturing environment environmental condition measurements are the one or more nominal environmental conditions.

FIELD OF THE INVENTION

The present invention relates generally to the field of magneticinformation storage and retrieval, and more particularly toautomatically compensating for manufacturing environmental changes whenwriting servo patterns to magnetic tape.

BACKGROUND OF THE INVENTION

As densities for linear tape storage systems increase due toadvancements in materials and storage schemes, precision alignment ofthe tape heads to data and servo tracks is an increasingly challengingrequirement for accurate recording and reading of stored data. At hightrack densities, alignment is affected by changes in tape width due toenvironmental conditions, such as temperature and humidity, andmechanical stress to the tape, which can cause the lateral spacing ofthe data tracks in a data band to shrink or expand. Generally,increasing the temperature or humidity will cause expansion of the tapewidth, and applying longitudinal stress to the tape will cause tensionnarrowing of the tape width. Changes to the lateral spacing of the dataand servo tracks may result in a misalignment of the tape head elementson the tape head assembly, such as read and write heads, with the dataand servo tracks on the tape. Depending on the latitudinal density ofdata tracks and the coefficients of thermal and hygroscopic expansion ofthe tape substrate material, the degree of expansion under certainenvironmental conditions may cause the lateral spacing of the data andservo tracks under the read heads of the tape head assembly to expand tothe point where not all tracks can be read by the tape heads.

One method of adjusting for the effects of changes to tape width whilerecording and reading data is to change the relative spacing of the readand write elements with respect to the tape by adjusting the tape headazimuth angle. Another method is to change the longitudinal tension onthe tape, causing a corresponding change to the lateral dimension of thetape due to tension narrowing of the tape. To facilitate thisadjustment, a pair of servo tracks can be used to measure the physicalwidth of the tape relative to that of the two servo elements.

Correct read and write element spacing relative to the data tracks inthe presence of changing tape width may be maintained by a feedbackcontrol system. Such a control system may require both a method ofdetermining the relative spacing, and an established reference value, orservo set point. One method of determining the relative spacing makesuse of servo patterns written in the servo bands on either side of databands during manufacturing. The servo patterns typically consist ofmagnetic transitions with two different azimuthal slopes, such as achevron pattern, that are read by a pair of servo read heads located onthe tape head. The servo read head lateral-position relative to a servotrack is derived from the relative timing of pulses read by the servoread head while reading the servo pattern. Reference signals forfeedback control can then be chosen with respect to the spacing computedfrom the servo patterns. For example, the control system could adjustthe azimuth angle of the tape head assembly or the tape tension suchthat both servo read heads track in the center of their respective servopatterns, and thus properly adjust the read head spacing relative to thedata tracks. If the servo spacing changes while the tape is running inthe tape drive unit, the servo feedback control system can compensatefor these changes.

A number of factors can contribute to differences in servo patternspacing. During manufacturing of the tape, the pattern width may beaffected by the environmental conditions during the writing of the servopatterns. Further variation may be introduced by tolerances inconstructing the servo writer head, which will also lead to differencesin servo pattern spacing. Additionally, aging of the tape causes theservo pattern spacing to change over time. This change is typicallynon-uniform due to the differences in pack pressure, or how tightlywound the tape is, within the tape cartridge. For example, tape at theinner diameters of the cartridge typically become wider than tape at theouter diameters due to these differences in pack pressure.

SUMMARY

In one embodiment, a first method for setting a servo write head spacingfor writing servo tracks on a magnetic tape in a manufacturingenvironment is disclosed. One or more processors receive one or moreenvironmental condition measurements of the manufacturing environment.The processors determine a spacing for a pair of servo write heads basedat least on a nominal spacing for the pair of servo tracks, and aproduct of a difference between one or more nominal environmentalconditions associated with the nominal spacing and corresponding one ormore of the environmental condition measurements of the manufacturingenvironment, and one or more coefficients of expansion for the magnetictape. Spacing for the pair of servo write heads is set to the determinedspacing, such that the spacing of the pair of servo write heads issubstantially the nominal spacing when the one or more manufacturingenvironment environmental condition measurements are the one or morenominal environmental conditions.

In another embodiment, a second method for setting a servo write headspacing for writing servo tracks on a magnetic tape in a manufacturingenvironment is disclosed. One or more processors receive one or moreenvironmental condition measurements of the manufacturing environment. Aspacing of a pair of servo write heads is set to a nominal spacing. Theone or more environmental condition measurements of the manufacturingenvironment, and one or more coefficients of expansion for the magnetictape that are associated with the one or more environmental conditionmeasurements of the manufacturing environment are recorded in one ormore data stores.

In another embodiment, a tape drive system is disclosed. One or moreenvironmental condition sensors generate one or more signalscorresponding to a temperature and/or humidity in an operatingenvironment of a pair of servo read heads of a tape drive system. Acontroller operates to read data associated with a magnetic tape, todetermine a spacing set point for the pair of servo read heads based atleast on the data and the one or more signals from the environmentalsensors, and to generate a control signal corresponding to thedetermined set point. An actuator coupled to the pair of servo readheads to adjusts the spacing of the pair of servo read heads to the setpoint based on the control signal.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a simplified component view of tape drive, in accordance withan embodiment of the present invention.

FIG. 2 is a side view of a tape head of the tape drive of FIG. 1, inaccordance with an embodiment of the present invention.

FIG. 2A is a plan view of a tape bearing surface of the tape head ofFIG. 2, in accordance with an embodiment of the present invention.

FIG. 2B is a detailed view of the read and/or write heads of the tapehead of FIG. 2, in accordance with an embodiment of the presentinvention.

FIGS. 3A-3C illustrate how tape lateral expansion and contraction may becompensated for by adjusting azimuth angle of a tape head, in accordancewith an embodiment of the present invention.

FIG. 4 is a flowchart illustrating operational steps of a servo writehead compensation system, in accordance with an embodiment of thepresent invention.

FIG. 5 is a flowchart illustrating operational steps of a second servowrite head compensation system, in accordance with an embodiment of thepresent invention.

FIG. 6 illustrates a simplified block diagram of a servo control systemfor controlling the spacing of servo write heads, in accordance with anembodiment of the present invention.

FIG. 7 illustrates a simplified block diagram of a second servo controlsystem for controlling the spacing of servo write heads, in accordancewith an embodiment of the present invention.

DETAILED DESCRIPTION

The following description is made for the purpose of illustrating thegeneral principles of the present invention and is not meant to limitthe inventive concepts claimed herein. Further, particular featuresdescribed herein can be used in combination with other describedfeatures in each of the various possible combinations and permutations.

An embodiment of the present invention generally describes a method andapparatus for setting the spacing of servo write heads such that thereis a nominal lateral spacing between the servo tracks at the nominaloperating temperature and humidity associated with the nominal spacing.Coefficients of thermal and hygroscopic expansion for the tape and tapehead substrate materials, and the manufacturing environment temperatureand humidity, may be used to determine the servo write head spacingduring manufacturing such that at the nominal temperature and humidity,the servo track spacing will be at the nominal value. The nominal servotrack width, the tape coefficients, and the tape manufacturing date arewritten to, for example, the tape, or media auxiliary memory of the tapecartridge containing the tape. At data read/write in a targetenvironment, a tape drive determines a servo set point based on thenominal servo track width, the difference between the nominaltemperature and humidity and the target operating environmenttemperature and humidity, the tape coefficients, and tape headcoefficients.

In another embodiment, the servo write head spacing is set to, forexample, the nominal lateral spacing regardless of the manufacturingenvironment temperature and humidity. The information related to thenominal servo track width, the manufacturing environment temperature andhumidity, the tape and tape head coefficients, and the tapemanufacturing date may be written to, for example, the tape, or mediaauxiliary memory of the tape cartridge containing the tape. At dataread/write in a target environment, a tape drive determines a servo setpoint based on the nominal servo track width, the difference between themanufacturing environment temperature and humidity and the targetoperating environment temperature and humidity, and the tape and tapehead coefficients.

FIG. 1 illustrates a simplified tape drive 100 of a tape-based datastorage system, in accordance with an embodiment of the presentinvention. While one specific implementation of a tape drive is shown inFIG. 1, it should be noted that the embodiments described herein may beimplemented in the context of any type of tape drive system.

As shown, tape supply cartridge 120 and take-up reel 121 are provided tosupport a tape 122. One or more of the reels may form part of aremovable cassette and are not necessarily part of tape drive 100. Tapedrive 100 typically includes drive motors (not shown) to drive tapesupply cartridge 120 and take-up reel 121 to move tape 122 over a tapehead 126 of any type.

Guides 125 guide tape 122 across tape head 126. Tape head 126 is in turncoupled to a tape controller 128 via a cable 130. In exemplaryembodiments, tape controller 128 at least controls certain functions oftape head 126, such as servo track writing during tape manufacturing,servo following, data writing, data reading, etc. Cable 130 may includeread/write circuits to transmit data to tape head 126 to be recorded onthe tape 122 and to receive data read by tape head 126 from the tape122. An actuator 132 controls the positioning of tape head 126 relativeto tape 122.

An interface 134 may also be provided for communication between tapedrive 100 and a host computing system (integral or external) to send andreceive data and for controlling the operation of the tape drive andcommunicating the status of the tape drive to the host computing system,as will be understood by those of skill in the art.

Tape drive 100 may also include environmental sensors (not shown) thatmonitor the temperature and humidity in the environment of tape head126. For example, sensors may be placed in proximity to tape head 126,or within an enclosure that contains tape drive 100. The sensors maycommunicate, for example, to the host computing system via interface134.

FIG. 2 illustrates a side view of an exemplary embodiment of tape head126 of FIG. 1, in accordance with an embodiment of the presentinvention. In the exemplary embodiment, tape head 126 is a flat-lapped,bi-directional, two-module magnetic tape head. Note that in variousembodiments of the present invention, other configurations may be used,which themselves may include components similar to and/or different thanthose shown in FIG. 2, as will be apparent to one skilled in the art.Tape head 126 includes a pair of modules, each comprised of a base 202,bonded to a chip 204. Each chip 204 includes a substrate 204A and aclosure 204B with a thin film portion, commonly referred to as a “gap”in which the read and/or write heads 206 are formed. The bases 202 aretypically “U-beams” which provide space for a cable in the center andallow adhesive bonding near the edges of the U. The U-Beam bases arebonded together at a small angle α with respect to each other. Afterbonding, the two modules form a single physical unit to provideread-while-write capability by activating the write heads of the leadingmodule and read heads of the trailing module aligned with the writeheads of the leading module parallel to the direction of tape travel. Inoperation, tape 122 is moved over chips 204 along tape bearing surfaces209 for reading and writing data on tape 122 using read and/or writeheads 206. The wrap angle β of tape 126 at edges going onto and exitingthe flat tape bearing surfaces 209 are usually between ⅛ degree and 4½degrees. Substrates 204A and closures 204B of chip 204 are typicallyconstructed of a wear resistant material, such as a ceramic.

FIG. 2A illustrates a plan view of tape bearing surface 209 of a chip204 from the perspective of looking down upon tape bearing surface 209of FIG. 2. A representative tape 122 is shown in dashed lines. In thisexemplary embodiment, tape 122 includes four data bands 208, and fiveservo tracks 210 on a one-half inch wide tape 122. Data bands 208 aredefined between servo tracks 210. Each data band 208 may include anumber of data tracks, for example 96 data tracks (not shown). Duringread/write operations, read and/or write heads 206 are positioned withinone of the data bands 208. Servo read heads read servo tracks 210.During tape manufacturing, servo write heads write the servo tracks 210to tape 122. Servo signals generated when servo read heads read servotracks 210 may be used to keep read and/or write heads 206 aligned withdata tracks in a particular data band 208 during read/write operations.Chip 204 is preferably long enough to be able to support tape 122 as thehead steps between data bands.

FIG. 2B depicts a plurality of read and/or write heads 206 formed in agap 218 on a chip 204 in Circle 2B of FIG. 2A. As shown, the array ofread and/or write heads 206 includes, for example, 16 data write heads214, 16 data read heads 216, two servo read heads 212, and two servowrite heads 220, though the number of elements may vary. While the readand/or write heads 206 may be arranged in a piggyback configuration asshown in FIG. 2B, data read heads 216 and data write heads 214 may alsobe arranged in an interleaved configuration. Alternatively, each arrayof read and/or write heads 206 may be data read heads or data writeheads only, and the arrays may contain one or more servo read heads 212or servo write heads 220. As noted by considering FIGS. 2, 2A, and 2Btogether, each chip 204 may include a complementary set of read and/orwrite heads 206 for such things as bi-directional reading and writing,read-while-write capability, backward compatibility, etc. In general,the data read heads 216 on one chip 204 are aligned with the data writeheads 214 on the opposite chip 204, such that data written by one modulecan subsequently be read by the second module as the tape moves from onemodule to the second module. Any of these arrays may contain one or moreservo read heads 212.

FIGS. 3A, 3B, and 3C illustrate how tape lateral expansion andcontraction may be compensated for by adjusting azimuth angle θ of atape head 302, in accordance with an embodiment of the presentinvention. FIGS. 3A, 3B, and 3C show tape 122 with servo tracks 210 anddata tracks 300, and tape head 302 with read and/or write heads 308,showing in this example servo read or write heads 304 and data readheads 306. As indicated, the direction of tape travel is longitudinallyin the direction of the data and servo tracks, and tape head 302 isdisposed predominantly laterally across tape 122.

In various embodiments, compensating for tape lateral expansion andcontraction is achieved by adjusting azimuth angle θ of tape head 302with respect to the latitudinal axis of tape 122, thereby altering therelative spacing of read and/or write heads 308, as projected in thedirection of tape travel of the tape over read and/or write heads 308.Generally, the projected relative spacing of read and/or write heads 308is governed by the product of the cosine of θ and the actual spacing ofread and/or write heads 308 on tape head 302. In one embodiment, tocompensate for tape lateral expansion and contraction, tape head 302 isset to a nominal azimuth angle θ_(nom) based, for example, on a nominalservo track and data track spacing defined in a specification orstandard associated with the tape technology. Adjustments may be made toazimuth angle θ to keep the projected relative spacing of read and/orwrite heads 308 aligned with servo tracks 210 and data tracks 300 oftape 122. This solution is represented for a single chip 204 in FIGS.3A, 3B, and 3C.

FIG. 3A shows tape head 302 relative to tape 122, where the tape has anominal width. As shown, servo read or write heads 304 and data readheads 306 are aligned with servo tracks 210 and data tracks 300,respectively, on tape 122. Tape head 302 is at an azimuth angle θ_(nom).FIG. 3B shows tape head 302 adjusted to an angle greater than azimuthangle θ_(nom) to compensate for tape lateral contraction. FIG. 3C showstape head 302 adjusted to an angle less than azimuth angle θ_(nom) tocompensate for tape lateral expansion. In the embodiment illustrated inFIG. 2, which includes two chips 204, each having a tape head assembly,a coupling arrangement between the two chips 204 may be used to ensurethat both modules maintain the same azimuth angle when adjustments tothe angle are made.

FIG. 4 is a flowchart illustrating operational steps of a servo writehead compensation system, in accordance with an embodiment of thepresent invention. In this embodiment, a tape is loaded into a tapedrive for a servo track write operation. The tape drive is typically adedicated servo track writer. The servo tracks are typically writtenonto tape that is stored on large reels, and is then spooled into, forexample, smaller tape cartridges. In the embodiment, the servo trackwriter system may receive several pieces of manufacturing set-up data.The system may receive data corresponding to the coefficients of thermaland hygroscopic expansion for the tape 122 (step 400). This data may beretrieved, for example, from a data store associated with the tapemanufacturing process, for example, a data store on the host computer,or may be hard coded in the system. This information is typicallydefined by the standard associated with the tape technology. Similarly,the system may receive data corresponding to the coefficients of thermaland hygroscopic expansion for the substrate of tape head 126 (step 402).In certain embodiments, the coefficient of hygroscopic expansion for thesubstrate of the tape head may be ignored because the substrate mayabsorb little to no moisture, and this coefficient can be treated aszero. The system also receives environmental data, for example, thetemperature and humidity, for the manufacturing environment (step 404).This data is received, for example, from the environmental sensors oftape drive 100.

The system also receives, or has hard coded information about, the servotrack width specification for tape 122 (step 406). The servo track widthis typically the nominal servo track spacing defined in a specificationor standard associated with the tape technology. In a preferredembodiment, the servo track width specification includes the associatedtemperature and humidity at which the nominal servo track spacing ismeasured.

In this embodiment, after the manufacturing set-up data has beenreceived, a servo write head spacing is determined such that the servotrack spacing will be at the nominal value when the tape is brought to anominal temperature and humidity. During servo track writing, the servowrite head spacing may be set to the determined value by, for example,adjusting the azimuth angle of write head 126 (step 408). For example,if the temperature and humidity of the manufacturing environment is thesame as the nominal temperature and humidity at which the nominal servotrack spacing is measured, the servo write head spacing can be set tothe nominal servo track spacing defined in the specification or standardassociated with the tape technology. If the temperature and humidity ofthe manufacturing environment is different than the nominal temperatureand humidity, the change in the width dimension of the tape and thelongitudinal dimension of the tape head resulting from the difference intemperature and humidity is determined, based on the received tape andtape head coefficients, and adjustments are made to the servo write headspacing, for example, by adjusting the azimuth angle of tape head 126.In certain embodiments, the servo track spacing may be adjusted byadjusting the tension of the tape at the servo write heads. The extentof adjustment to the servo track spacing on a tape may be determined,for example, by Poisson's ratio for the tape, and the maximum allowabletension set in the tape specification.

For example, the differences between the temperature and humidity of themanufacturing environment and nominal temperature and humidity may berepresented by the following equations:ΔT=(mfg temp)−(nominal temp)ΔH=(mfg humidity)−(nominal humidity)

The dimensional change in the tape servo track spacing between themanufacturing environment and an environment at the nominal temperatureand humidity may be represented by the following equation:ΔL _(TAPE) =ΔT(tape coeff(T))+ΔH(tape coeff(H))where “tape coeff(T)” and “tape coeff(H)” represent the coefficients ofthermal and hygroscopic expansion, respectively, of the tape substrate,and the coefficients have dimensions of [length/temperature] and[length/humidity], respectively.

Similarly, the dimensional changes in the target tape head substratebetween the manufacturing environment and an environment at the nominaltemperature and humidity may be represented by the following equation:ΔL _(HEAD) =ΔT(target tape head coeff(T))+ΔH(target tape head coeff(H))

A servo write head spacing may be determined by the following equation:Servo Spacing=(Nominal Spacing)+ΔL _(TAPE) −ΔL _(HEAD)

Thus, a servo write head spacing can be based on the nominal spacing,adjusted for the differential difference in dimensional changes to thetape and the manufacturing tape drive tape head due to the differencebetween the temperature and humidity of the target environment and thenominal temperature and humidity.

In certain implementations, the environment in which a tape has beenstored awaiting the writing of servo tracks, in relation to the time thetape is in the manufacturing environment of tape drive 100, may have ahigher importance in determining the servo write head spacing. Forexample, if a tape has reached an equilibrium state with respect to thetemperature and humidity of a storage environment and the tape is thenmoved to a manufacturing environment with a different temperature andhumidity from the storage environment, but one in which the tape will beexposed to for only a short period of time during the writing of theservo tracks, the temperature and humidity of the storage environmentmay have a higher weighting in determining the servo write head spacingthan the temperature and humidity of the manufacturing environment.

After the servo write head spacing has been set (see step 408), servotracks are written to the tape (step 410). During the writing of theservo tracks, the azimuth angle (and/or the tape tension at the servowrite head) may be monitored (step 412). If deviations from thedetermined azimuth angle (or tape tension) are detected, adjustments tothe servo write head spacing may be made as needed (step 414).

As part of the servo track writing process, various data related to theprocess may optionally be recorded to a data store (step 416). In oneembodiment, the following data may be written: the nominal servo trackspacing and the nominal temperature and humidity; the tape coefficientsof temperature and humidity; and the tape manufacturing date. The datamay be recorded to the media auxiliary memory of a tape cartridge, suchas the cartridge memory of the Linear Tape-Open (LTO) standard formagnetic tape data storage. Alternatively, the data may be recorded to adata structure on the tape itself, for example, to the tape header or toa special purpose dataset on the tape, or stored in a dataset on acomputer-readable tangible storage medium on a host computing system andaccessible to a tape drive via, for example, an interface, such asinterface 134. In certain embodiments, the nominal servo track spacing,the nominal temperature and humidity, and the tape coefficients oftemperature and humidity are not written to the tape or tape cartridgebecause these values may be defined by the standard associated with thetape technology, and may be, for example, hard coded in the system, orstored in a data store of the system.

In certain embodiments, the manufacturing environmental conditions maybe continuously monitored (see, e.g., step 404), and if changes aredetected during the servo track writing operation, adjustments can bemade to the servo write head spacing set point (see, e.g., step 408),via, for example, the servo write head azimuth angle, to compensate forchanges in tape dimension resulting from the changes in manufacturingenvironmental conditions.

FIG. 5 is a flowchart illustrating operational steps of a second servowrite head compensation system, in accordance with another embodiment ofthe present invention. In this embodiment, when a servo track writeoperation is requested on a servo track writer, the system may receiveseveral pieces of data. The system may receive data corresponding to thecoefficients of thermal and hygroscopic expansion for the tape 122 (step500). This data may be retrieved, for example, from a data storeassociated with the tape manufacturing process, for example, a datastore on the host computer, or may be hard coded in the system. Thisinformation is typically defined by the standard associated with thetape technology. Similarly, the system may receive, or have hard codedinformation about, data corresponding to the coefficients of thermal andhygroscopic expansion for the substrate of tape head 126 (step 502). Asdescribed above, the coefficient of hygroscopic expansion for thesubstrate of the tape head may be ignored, as it typically is very closeto zero. While this data is not needed during the tape servo trackwriting process, it may optionally be written to the tape or tapecartridge, as described below.

The system also receives environmental data, for example, thetemperature and humidity, for the manufacturing environment (step 504).This data is received, for example, from the environmental sensors oftape drive 100. The system may also receive, or have hard codedinformation about, the servo track width specification for tape 122(step 506). The servo track width is typically the nominal servo trackspacing defined in a specification or standard associated with the tapetechnology. In a preferred embodiment, the servo track widthspecification includes the associated temperature and humidity at whichthe nominal servo track spacing is measured.

In this embodiment, the servo write head spacing is set to the nominalvalue, regardless of the manufacturing environment temperature andhumidity, by, for example, adjusting the azimuth angle of write head 126(step 508). In other embodiments, the servo write head assembly may, forexample, be fixed at the nominal spacing, for example, bolted on place.After the servo write head spacing has been set, servo tracks arewritten to the tape (step 510). As part of the servo track writingprocess, various data related to the process is recorded to a data store(step 512). In this exemplary embodiment, the following data may bewritten: the nominal servo track spacing and the nominal temperature andhumidity; the manufacturing environment temperature and humidity; thetape and tape head coefficients of temperature and humidity; and thetape manufacturing date. The data may be recorded to the media auxiliarymemory of a tape cartridge, such as the cartridge memory of the LinearTape-Open (LTO) standard for magnetic tape data storage. Alternatively,the data may be recorded to the tape itself, for example, to the tapeheader or to a special purpose dataset on the tape, or stored in adataset on a computer-readable tangible storage medium on a hostcomputing system and accessible to a tape drive via, for example, aninterface, such as interface 134.

FIG. 6 illustrates a simplified block diagram of a servo control system600 for controlling the writing of servo tracks to a tape, in accordancewith an embodiment of the present invention as described with relationto the flowchart of FIG. 4. In this servo control system, spacing ofservo write heads during the tape manufacturing process is adjusted fromthe nominal spacing based on differences between the nominal temperatureand humidity and the manufacturing environment temperature and humidity(see step 408, FIG. 4). Initial servo write head spacing is determinedby a reference signal R derived from the manufacturing environmentaldata, such as the temperature and humidity, the temperature and humiditycoefficients of expansion for the tape and tape head, the nominal widthof the servo tracks, and the associated nominal temperature andhumidity.

Based on reference signal R, controller 602 generates a control signalU_(control) to actuator 604 to set, for example, the azimuth angle ofthe tape head of head module 606 such that the servo head spacing is setto, for example, the nominal spacing. An azimuth angle detector 608 maymonitor the azimuth angle of tape head module 606, and provide afeedback signal s(t) that indicates deviation from the desired azimuthangle. Signal s(t) is combined with reference signal R via subtracter610 to generate azimuth angle error signal e(t), which is used bycontroller 602 to make required adjustments to, for example, the tapehead azimuth angle of head module 606 via control signal U_(control) toactuator 604. In certain embodiments, setting and adjusting of the tapeservo track spacing as the servo tracks are written to the tape may becontrolled, for example, by one or both of adjusting the azimuth angle,and tape tension.

In certain embodiments, the manufacturing environmental conditions canbe continuously monitored, and detected changes may be used to adjustreference signal R.

When a tape manufactured in accordance with an embodiment described inrelation to FIGS. 4 and 6 is loaded into a tape drive, for example, tapedrive 100, for routine data write and read operations, the tape drivecan, for example, determine a servo head spacing set point, based on thenominal servo track spacing and the associated nominal temperature andhumidity, and adjustments to the nominal spacing based on tapecoefficients, coefficients of the tape head that is reading the tape,and differences between the temperature and humidity in the targetenvironment at which the servo tracks are read from the tape and thenominal temperature and humidity. As mentioned above, the nominal servotrack spacing, the associated nominal temperature and humidity, and thetape coefficients, are typically associated with a tape standard. Thetape drive, for example, tape drive 100, may include hard codedinformation regarding these values, these values may be stored on acomputer readable tangible storage device, these values may be writtento the tape or tape cartridge, or a combination of these.

For example, the differences between the temperature and humidity of thetarget environment in which, for example, tape 100 operates, and thenominal temperature and humidity may be represented by the followingequations:ΔT=(target temp)−(nominal temp)ΔH=(target humidity)−(nominal humidity)

The dimensional change in the tape servo track spacing between thetarget environment and an environment at the nominal temperature andhumidity may be represented by the following equation:ΔL _(TAPE) =ΔT(tape coeff(T))+ΔH(tape coeff(H))where “tape coeff(T)” and “tape coeff(H)” represent the coefficients ofthermal and hygroscopic expansion, respectively, of the tape substrate,and the coefficients have dimensions of [length/temperature] and[length/humidity], respectively.

Similarly, the dimensional changes in the target tape head substratebetween the target environment and an environment at the nominaltemperature and humidity may be represented by the following equation:ΔL _(HEAD) =ΔT(target tape head coeff(T))+ΔH(target tape head coeff(H))

A servo read head spacing set point may be determined by the followingequation:Set Point=(Nominal Spacing)+ΔL _(TAPE) −ΔL _(HEAD)  (1)

Thus, a set point can be based on the nominal spacing, adjusted for thedifferential difference in dimensional changes to the tape and thetarget tape drive tape head due to the difference between thetemperature and humidity of the target environment and the nominaltemperature and humidity.

FIG. 7 illustrates a simplified block diagram of a servo control system700 that may be used for controlling the servo write head spacing, inaccordance with an embodiment of the present invention as described withrelation to the flowchart of FIG. 5. In this open loop servo controlsystem, the servo write head spacing is set to the nominal value,regardless of the manufacturing environment temperature and humidity,by, for example, adjusting the azimuth angle of write head 126 (see step508, FIG. 5). Initial servo write head spacing is determined by areference signal R derived from the nominal servo track width. Based onreference signal R, controller 702 generates a control signalU_(control) to actuator 704 to set, for example, the azimuth angle ofthe tape head of head module 706 such that the servo write head spacingis set to the nominal spacing. As mentioned above, in certainembodiments, the servo write heads may be fixed in place at the nominalservo track width, and a servo control system is not used.

When a tape manufactured in accordance with an embodiment described inrelation to FIGS. 5 and 7 is loaded into a tape drive, for example, tapedrive 100, for routine data write and read operations, the tape drivecan, for example, determine a servo head spacing set point, based on thenominal servo track spacing and the associated nominal temperature andhumidity, and adjustments to the nominal spacing based on tapecoefficients, coefficients of the manufacturing tape head and the tapehead that is reading the tape, and differences between the temperatureand humidity in the target environment at which the servo tracks areread from the tape and the manufacturing temperature and humidity atwhich the servo tracks were written. As mentioned above, the nominalservo track spacing, the associated nominal temperature and humidity,and the tape coefficients, are typically associated with a tapestandard. The tape drive, for example, tape drive 100, may include hardcoded information regarding these values, these values may be stored ona computer readable tangible storage device, these values may be writtento the tape or tape cartridge, or a combination of these.

For example, similar to the derivation of equation (1) above, thedifferences between the temperature and humidity of the environmenttarget and manufacturing temperature and humidity may be represented bythe following equations:ΔT=(target temp)−(mfg temp)ΔH=(target humidity)−(mfg humidity)

The dimensional changes in the tape servo track spacing and thedimensional changes in the target tape head substrate between the targetenvironment and the manufacturing environment may be represented by thefollowing equations:ΔL _(TAPE) =ΔT(tape coeff(T))+ΔH(tape coeff(H))ΔL _(HEAD) =ΔT(target tape head coeff(T))+ΔH(target tape head coeff(H))

Similar to above, a servo read head spacing set point may be determinedby the following equation:Set Point=(Nominal Spacing)+ΔL _(TAPE) −ΔL _(HEAD)  (2)

In certain embodiments, a servo read head spacing set point may also bebased on the age of the tape, determined, for example, from the date ofmanufacture recorded in a data store, and the corresponding non-uniformeffect on tape width, as a function of tape longitudinal position,caused by pack pressure. As described above, as a tape ages, tape at theinner diameters of the cartridge typically becomes wider than tape atthe outer diameters due to differences in pack pressure. This effect isnon-linear with respect to age, and tends to stabilize after a period oftime. For example, an empirical relationship can be determined betweenthe age of a tape and the change in tape width as a function of tapelongitudinal position caused by pack pressure, by examining tapes ofdifferent ages. Tapes having different tape materials and compositionsmay exhibit varying effects due to pack pressure. Changes in width dueto pack pressure may be added to the right side of equations (1) or (2)to arrive at a set point value.

As will be appreciated by one skilled in the art, aspects of the presentinvention may be embodied as a system or method. Accordingly, aspects ofthe present invention may take the form of an entirely hardwareembodiment, an entirely software embodiment (including firmware,resident software, micro-code, etc.) or an embodiment combining softwareand hardware aspects that may all generally be referred to herein as a“circuit,” “module,” or “system.”

Any flowcharts and block diagrams in the Figures illustrate thearchitecture, functionality, and operation of possible implementationsof systems, methods and computer program products according to variousembodiments of the present invention. In this regard, each block in aflowchart or block diagram may represent a module, segment, or portionof code, which comprises one or more executable instructions forimplementing the specified logical function(s). It should also be notedthat, in some alternative implementations, the functions noted in theblock may occur out of the order noted in the figures. For example, twoblocks shown in succession may, in fact, be executed substantiallyconcurrently, or the blocks may sometimes be executed in the reverseorder, depending upon the functionality involved. It will also be notedthat each block of the block diagrams and/or flowchart illustration, andcombinations of blocks in the block diagrams and/or flowchartillustration, can be implemented by special purpose hardware-basedsystems that perform the specified functions or acts, or combinations ofspecial purpose hardware and computer instructions.

The foregoing description of various embodiments of the presentinvention has been presented for purposes of illustration anddescription. It is not intended to be exhaustive nor to limit theinvention to the precise form disclosed. Many modifications andvariations are possible. Such modifications and variations that may beapparent to a person skilled in the art of the invention are intended tobe included within the scope of the invention as defined by theaccompanying claims.

What is claimed is:
 1. A method for setting a servo write head spacing for writing servo tracks on a magnetic tape in a manufacturing environment, the method comprising: receiving, by one or more processors, one or more environmental condition measurements of the manufacturing environment; determining, by the one or more processors, a spacing for a pair of servo write heads based at least on: a nominal spacing for the pair of servo tracks; a difference between one or more nominal environmental conditions associated with the nominal spacing and corresponding one or more of the environmental condition measurements of the manufacturing environment; and one or more coefficients of expansion for the magnetic tape; and setting the spacing for the pair of servo write heads to the determined spacing; whereby the spacing of the pair of servo write heads is substantially the nominal spacing when the one or more manufacturing environment environmental condition measurements are the one or more nominal environmental conditions.
 2. A method in accordance with claim 1, wherein the coefficients of expansion for the magnetic tape include one or more of: a coefficient of thermal expansion; and a coefficient of hygroscopic expansion.
 3. A method in accordance with claim 1, wherein determining, by the one or more processors, the spacing of the pair of servo write heads is further based at least on one or more coefficients of expansion for a substrate of the servo write heads.
 4. A method in accordance with claim 3, wherein the coefficients of expansion for the substrate of the servo write heads includes one or more of: a coefficient of thermal expansion; and a coefficient of hygroscopic expansion.
 5. A method in accordance with claim 1, further comprising recording in one or more data stores one or more of: the one or more coefficients of expansion for the magnetic tape, the nominal spacing for the pair of servo tracks, the nominal environmental conditions associated with the nominal spacing, and the date of writing the pair of servo tracks on the tape.
 6. A method in accordance with claim 5, wherein a data store is one of: media auxiliary storage of a tape cartridge of the tape; a data structure on the tape; and a dataset on a computer-readable tangible storage medium on a host computing system.
 7. A method in accordance with claim 1, wherein setting the spacing of the pair of servo write heads comprises setting an azimuth angle of a tape head that includes the pair of servo write heads.
 8. A magnetic tape system comprising: one or more environmental condition sensors to generate one or more signals corresponding to a temperature and/or humidity in an operating environment of a pair of servo read heads of a magnetic tape system; a controller operated to read data associated with a magnetic tape from a data store, to determine a spacing set point for the pair of servo read heads based at least on the data and the one or more signals from the environmental sensors, and to generate a control signal corresponding to the determined set point; and an actuator coupled to the pair of servo read heads to adjust the spacing of the pair of servo read heads to the set point based on the control signal.
 9. A magnetic tape system in accordance with claim 8, wherein the data associated with the magnetic tape includes one or more of: a coefficient of thermal expansion for the magnetic tape; a coefficient of hygroscopic expansion for the magnetic tape; a coefficient of thermal expansion for a substrate of a tape head containing the pair of servo read heads; a coefficient of hygroscopic expansion for a substrate of the tape head containing the pair of servo read heads; a nominal spacing value for the pair of servo read heads and a nominal temperature and/or humidity associated with the nominal spacing value; a temperature and/or a humidity of a manufacturing environment in which a pair of servo tracks was written to the tape.
 10. A magnetic tape system in accordance with claim 8, wherein determining a spacing set point for the pair of servo read heads comprises: determining, by the controller, a spacing set point for the pair of servo read heads based at least on: the nominal spacing value for the pair of servo read heads; and a product of a difference between the nominal temperature and/or humidity associated with the nominal spacing value and the corresponding temperature and/or humidity of the operating environment of the pair of servo read heads of the magnetic tape system, and the corresponding coefficient(s) of thermal and/or hygroscopic expansion for the magnetic tape.
 11. A magnetic tape system in accordance with claim 8, wherein determining a spacing set point for the pair of servo read heads comprises: determining, by the controller, a spacing set point for the pair of servo read heads based at least on: the nominal spacing value for the pair of servo read heads; and a product of a difference between the temperature and/or a humidity of a manufacturing environment in which a pair of servo tracks was written to the tape and the corresponding temperature and/or humidity of the operating environment of the pair of servo read heads of the magnetic tape system, and the corresponding coefficient(s) of thermal and/or hygroscopic expansion for the magnetic tape.
 12. A magnetic tape system in accordance with claim 8, wherein a data store is one or more of: a media auxiliary storage of a tape cartridge of the tape; a data structure on the tape; and a dataset on a computer-readable tangible storage medium on a host computing system.
 13. A magnetic tape system in accordance with claim 8, wherein the magnetic tape system is one of a tape drive system, and a servo writing system. 